1. Field of the Invention
The invention relates to methods and apparatus for the measurement of volume holographic gratings.
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2. Background Art
Holography is the process of recording phase information into a material that is sensitive to the intensity of the incident illumination (“Introduction to Fourier Optics”, J. W. Goodman, McGraw-Hill, 1968). Early holographic recording materials were primarily photographic films, but modern photorefractive materials additionally include dichromated gelatin films, LiNbO3 and other crystals, polymers, and glasses. Amplitude and phase information can be recorded through the interference of mutually coherent signal and reference beams.
When the signal and reference beams are simple plane waves, the material records the single sinusoidal intensity pattern formed by their interference. The grating is referred to by its grating vector, which has a grating magnitude and orientation. The magnitude is the refractive index modulation depth for materials that contain phase gratings, and is the absorption modulation for materials that contain amplitude gratings. The orientation is determined by the angle between the recording beams and the recording material. If the signal or reference beam, or both, are not simple plane waves but rather carry information in the form of phase or intensity variations, then the recorded hologram can be thought of as being composed of many superimposed individual gratings each recorded by pairs of plane waves from the Fourier decomposition of the recording beams. A description of this process is found in the reference by J. W. Goodman noted above.
Holographic recording can be used with thin or thick media. When the material in which the hologram is present is thick, then Bragg selectivity occurs (“Coupled Wave Theory for Thick Hologram Gratings”, H. Kogelnik, The Bell System Tech. J. 48:9, 1969). Volume hologram reflection gratings have been shown to be an extremely accurate and temperature-stable means of filtering a narrow passband of light from a broadband spectrum. This technology has been demonstrated in practical applications where narrow full-width-at-half-maximum (FWHM) passbands are required. Furthermore, such filters have arbitrarily selectable wavefront curvatures, center wavelengths, and output beam directions.
Photorefractive materials, such as LiNbO3 crystals and certain types of polymers and glasses, have been shown to be effective media for storing volume holographic gratings. Uses include optical filters or holographic optical memories with high diffraction efficiency and storage density (F. Havermeyer, W. Liu, C. Moser, D. Psaltis, G. J. Steckman, Volume Holographic Grating-Based Continuously Tunable Optical Filter, Optical Engineering 43(0), September 2004) (G. J. Steckman, A. Pu, and D. Psaltis, Storage Density Of Shift-Multiplexed Holographic Memory, Applied Optics 40 (20): 3387-3394 Jul. 10, 2001) (G. J. Steckman, R. Bittner, K. Meerholz, and D. Psaltis, Holographic Multiplexing In Photorefractive Polymers, Optics Communications 185(1-3): 13-17 Nov. 1, 2000) (G. J. Steckman, I. Solomatine, G. Zhou, and D. Psaltis, Holographic Data Storage In Phenanthrenequinone Doped PMMA, SPIE Photonics West, San Jose, Calif., Jan. 27, 1999) (M. Levene, G. J. Steckman, and D. Psaltis, Method For Controlling The Shift Invariance Of Optical Correlators, Applied Optics 38 (2): 394-398 Jan. 10, 1999) (U.S. Pat. No. 6,829,067). In addition, volume gratings Bragg-matched to reflect at normal incidence have been used successfully to stabilize and lock the wavelength of semiconductor laser diodes (U.S. Pat. No. 5,691,989).
FIG. 1 illustrates a non-slanted volume holographic grating 100 readout at an angle θ with readout beam 105. When the Bragg condition, λr=2n(λr)Λ cos(θr), is satisfied the grating will diffract the incident beam into the output beam 110 where λr is the readout wavelength in vacuum, n(λr) is the refractive index of the volume holographic grating element material at the readout wavelength, Λ is the grating spacing, and θ is the readout angle of incidence inside the material. The paper by H. Kogelnik describes volume holographic grating diffraction. For a simple uniform grating, the characteristics are determined by the thickness of the volume element, the refractive index modulation depth, the grating spacing, and the slant angle relative to the surface normal.
For mass production of volume holographic grating elements it is desirable to measure these characteristics at the wafer level before dicing into final parts of smaller size. Prior art measurement systems use a single readout laser beam that is detected by a single element, thereby integrating the performance over the area illuminated by the beam. The spatial resolution and area of test coverage is limited by the beam diameter. Increasing the area of coverage is possible by scanning a small beam over the area of a larger wafer, however this is a time consuming process, particularly if a high resolution, and therefore a small diameter readout beam, is required.